Optical observation instruments are used under various circumstances in which there is a danger that a flare of high intensity enters into the observation instrument in addition to the proper observation light. Such flare may endanger or damage the eyes of a user of the observation instrument and likewise endanger or damage an observation sensor element.
In the military field it is well known to provide visual observation instruments, i.e. telescopes, periscopes, night vision instruments, video cameras and the like with a device for protecting the observer against flare. This holds true in particular for laser radiation of high intensity as emitted in combat situations by enemy laser weapons but also by range finders or aiming devices. Conventionally, Nd-YAG solid state lasers having a wave length of about 1.0 μm, i.e. above the visible range of between 400 and 800 nm, are used for that purpose.
In this context U.S. Pat. No. 6,075,661 discloses an adapter that may be applied in front of a lens of an optical instrument, for example a night vision goggle. The adapter comprises an interference filter tuned to the laser light wave length.
U.S. Pat. No. 4,934,246 discloses a periscope for a tank vehicle. The periscope is provided with a window made from tempered glass and acting as a laser filter.
U.S. Pat. No. 4,909,609 describes a protection device for telescopes and the like utilizing a nonlinear optical element for protecting the observer against laser radiation of high intensity.
Further prior art devices utilize mechanical or electrooptical apertures or shutters.
Similar problems likewise arise in the civil sector. For example, in the field of ophthalmology laser light is used for diagnostic and therapeutic, in particular surgical purposes, with an optical observation taking place simultaneously. On the other hand, laser light is also used for various industrial manufacturing applications, for example for the production of semiconductor components, for marking spectacle lenses and the like. In this context Excimer lasers are conventionally used having a wavelength depending on the particular design and being 350 nmm (XeF), 248 nm (KrF) or 193 nm (ArF), i.e. below the visible range.
Under a more general aspect, the invention is also related to flare protection devices of other kind, for example on rear mirrors in motor vehicles.
The invention is particularly related to visual observation instruments but not limited to that field.
In an optical observation instrument of the type specified at the outset, i.e. a telescope for military applications, it is known in the art to apply a laser protection coating on a lens arranged within the beam path. In that case, the filter layer acts as an interference filter having a spectrally selective transmission behaviour. The flare transits the interference filter one time.
The disadvantage of this prior art device lies in the fact that it is impossible to manufacture filter layers having an ideally homogenous transmission distribution over the surface thereof. On the other hand, for a sufficient protection, the flare transmission should be of the order of 10−5. This can, however, only be achieved if the sum of the unavoidable defective spots within a filter layer as manufactured under real conditions, is correspondingly small. With an effective diameter of 10 mm, the defective spot surface fraction may not exceed 0.003 mm2. Such a low surface fraction, however, can only be achieved, if at all possible, when extreme cleanliness requirements are fulfilled during the production of the filter layer or when extreme scrap ratios are accepted.
Interference filters, moreover, have the disadvantage that due to the highly exact parallelism between the input and the output surfaces, on which the flare impinges under right angles, multiple reflections may occur between these surfaces. One can show that the optical density and, hence, the protective effect of the filter decreases with the number of the multiple reflections.
It is, therefore, an object underlying the invention to improve an optical observation instrument of the type specified at the outset such that the afore-mentioned disadvantages are avoided. In particular, an observation instrument shall be provided that has a high protective effect against flare while the flare filter may be manufactured at reasonable costs.